1. Field of the Invention
The present invention relates to an aromatic polycarbodiimide and a polycarbodiimide sheet. More particularly, the present invention relates to an aromatic polycarbodiimide having high heat resistance, low hygroscopicity and a low dielectric constant and to a sheet made from such an aromatic polycarbodiimide.
2. Description of the Related Art
There have been known aromatic polycarbodiimides produced by polymerizing diphenylmethane diisocyanate (MDI) or tolylene diisocyanate (TDI) as a monomer. These aromatic polycarbodiimides are used as flame resistant films or heat resistant adhesives because of their excellent heat resistance.
From the point of view that these aromatic polycarbodiimide films do not generate a volatile gas or decomposed monomer even when they are exposed to a temperature higher than 400.degree. C., it can be said that these aromatic polycarbodiimide films have acceptable heat resistance. However, they have low moisture resistance, and loses self supporting properties and become fragile when they are heat-treated at 200.degree. C. or more, so that they are not fit for practical use. In addition, they have poor solubility in an organic solvent and low workability.